Lymphocytes. T cells, B cells, and natural killer, or NK, cells.

 

Lymphocytes

• Lymphocytes are the major cells of the adaptive immune system.
• Lymphocytes can be divided into three populations:

                     1) T cells,

                     2) B cells, and

                     3) natural killer, or NK, cells.

• B and T lymphocytes differentiate from their respective

lymphoid precursor cells and

• leave the bone marrow in a kind of cellular stasis-not

actively replicating like other somatic cells .

• These cells are said to be naive.

 T- lymphocytes or T cells

• Lymphocytes destined to become T lymphocytes or T cells leave the bone marrow and

mature in the thymus gland.

• They can remain in the thymus, circulate in the blood, or reside in lymphoid organs such as the lymph nodes and spleen.
• Naïve T cells require a specific antigen to bind to a specific, membrane bound receptor (the T-cell receptor) to signal the continuation of replication.
• The now "activated" T cells differentiate into effector cells and memory cells.

     1. EffectorT cells include the

• T-helper cells (TH) ,
• cytotoxic lymphocytes (CTLs),
• natural killer T cells, or
• T-regulatory cells,
• that respond to a myriad of antigens by producing and secreting cytokines .
• The cytokines secreted from effector T cells control specific responses directing theactions of other host cells.

     2. Memory cells are quiescent, only to become activated T cells upon a subsequentexposure to the antigen.

• They provide a faster, heightened response.

B lymphocytes or B cells

• After B lymphocytes or B cells reach maturity within the bone marrow,
• they also circulate in the blood and
• disperse into various lymphoid organs where they await to become activated.
• The activated B cell becomes more ovoid.
• Its nuclear chromatin condenses, and numerous folds of endoplasmic reticulum become more visible.
• A mature, activated B cell is called a plasma cell.
• Plasma cells secrete large quantities of antibodies .
• Activated B cells also produce memory cells that are primed to act upon subsequent exposure to antigen

Natural killer (NK) cells

• Natural killer (NK) cells are a small population of large,

non phagocytic granular lymphocytes that play an important role in innate immunity.

• The major NK cell function is to attack and destroy
• malignant cells and
• cells infected with microorganisms, such as viruses and intracellular bacteria;
• however, NK cells do not recognize antigen.
• They recognize their targets in one of two ways
• First, NK cells survey somatic cells for a specific membrane-bound protein,known as the class I major histocompatibility complex (MHC) protein.

• Normal healthy host cells express class I MHC proteins on the cell surface, andNK cells bear a receptor that recognizes MHC class I.

• When NK cells encounter host cells with altered or missing class I MHC proteins, as sometimes happens during intracellular infection and oncogenesis, the aberrant cells are destroyed.
• NK cells also recognize and eliminate infected and malignant cells, expressing what are called stress, or shock, proteins on their surface.

• Finally, because NK cells have receptors for antibodies, they can also attack cells that are opsonized by antibodies. This process is called antibody dependent cell-mediated cytotoxicity (ADCC) and causes the death of the target cell.

• In all cases, attack by the NK cell results in the release of

pore-forming, perforin proteins and enzymes called granzymes.

• Together the perforins and granzymes induce target cell to commit suicide (apoptosis) .

• Unlike T and B cells, NK cells do not exhibit memory responses to target cells, thus they are considered part of the innate immune response.